Field of the Invention
This invention relates generally to digital imaging, and more particularly to high dynamic range (HDR) pixel arrays.
Description of the Background Art
Color sensitive digital imagers are well known. Typically color imagers are comprised of a plurality of image sensor pixels, each being sensitive to light of one of three different colors. The pixels are typically laid out in rows and columns, wherein each square of four pixels includes one pixel sensitive to two of the three colors and two pixels sensitive to the third color. Through this arrangement, color images are created by interpolating missing color information from neighboring pixels of that color. Color images created in this way have deficiencies in quality, which result from filtering out a large portion of the incident light and filling in missing color information, such as poor light sensitivity and low resolution.
One solution for increasing the quality of color images is the utilization of a big-small pixel scheme for laying out pixels on an imager sensor. In the big-small pixel scheme, some of the pixels are larger than others. In prior art systems, both big and small pixels are sensitive to one of three colors (e.g., R, G, B). The big pixels are more sensitive to lower light intensities than the small pixels. The big-small scheme facilitates the capture of images with a higher dynamic range (HDR) and provides more detail in images with widely varying local light intensities. In areas of the image with high light intensity, the small pixels will be sampled, because they are less sensitive to the light and will, therefore, be less likely to have become saturated. In areas of the image with lower light intensity, the big pixels will be sampled, because they are more sensitive to the light. The resultant image has a higher dynamic range, and shows more detail in both high and low light areas. However, big-small schemes are not preferable for low light applications, because a significant portion of the impinging light is filtered out by the color filters which pass only a small spectral band compared to the entire visible spectrum.
Another solution for increasing the quality of color images is the utilization of polychromatic (i.e. white) image sensing pixels in addition to the color sensing pixels. These pixels are sensitive to light across most of the visible spectrum, increasing the light sensitivity of the white pixels as compared to the color filtered pixels. Several white/color pixel layouts have been utilized, including a big-small scheme, in which all the small pixels are polychromatic (e.g., white) and all the big pixels are color sensitive (e.g., R, G, B). This layout has been used, for example to reduce color aliasing in a recorded image. Another layout utilizes one each of the three color sensitive pixels and a polychromatic pixel arranged in a square, which is repeated. Yet another layout utilizes hexagonal pixels to create a layout with twice as many polychromatic light sensing pixels as color sensing pixels. Yet another layout utilizes a 4×4 square of image sensing pixels wherein the color pixels are arranged along a diagonal of the square (G-R-G-B) and the remaining pixels of the square are polychromatic. These layouts each improve image resolution and sensitivity, but there is still a desire for even better image quality.
Yet another solution for increasing the quality of color images is the fusion of a monochromatic and color version of the same image, taken with separate image sensors. The monochromatic image, because all the pixels are the same, has a resolution that is at least three times higher than the color image. Using the monochromatic image as the base, the two are used to generate a resulting color image that has a higher resolution and lower noise than a traditional color image. Drawbacks to this solution include the increased costs and complexity due to the utilization of two separate imagers. This solution also requires that the two images be realigned digitally, which requires increased processing power and sacrifices speed. Additionally, this solution sacrifices low light sensitivity, because the two sensors have pixels of the same size.
What is needed, therefore, is a single color imager that produces images with an even greater sensitivity, resolution, and/or dynamic range than the prior art.